In order to address the issue of increasing bandwidth requirements that are demanded for wireless communications systems, different schemes are being developed to allow multiple user terminals to communicate with a single access point by sharing the channel resources while achieving high data throughputs. Multiple Input and Multiple Output (MIMO) technology represents one such approach that has recently emerged as a popular technique for next generation communication systems. MIMO technology has been adopted in several emerging wireless communications standards such as the Institute of Electrical Engineers (IEEE) 802.11 standard. IEEE 802.11 denotes a set of Wireless Local Area Network (WLAN) air interface standards developed by the IEEE 802.11 committee for short-range communications (e.g., tens of meters to a few hundred meters).
In wireless communications systems, Medium Access Control (MAC) protocols are designed to exploit several dimensions of freedom offered by the air link medium. The most commonly exploited dimensions of freedom are time and frequency. For example, in the IEEE 802.11 MAC protocol, the “time” dimension of freedom is exploited through the Carrier Sense Multiple Access (CSMA) protocol. The CSMA protocol attempts to ensure that no more than one transmission occurs during a period of potential high interference. Similarly, the “frequency” dimension of freedom may be exploited by using different frequency channels.
Recent developments have led to the space dimension being a viable option for increasing, or at least more efficiently using, existing capacity. Spatial Division Multiple Access (SDMA) may be used for improving the utilization of the air link by scheduling multiple terminals for simultaneous transmission and reception. In SDMA, data is sent to each of the terminals using spatial streams. A transmitter may form orthogonal streams to individual receivers such that a stream targeted at a given station STA-A, for example, is seen as low power interference at STA-B, STA-C, etc., which will not cause significant interference and most likely be ignored. Such orthogonal streams can be formed because the transmitter has several antennas and the transmit/receive channel includes several paths. Receivers may also employ MIMO technology.
Additional complexities arise when MIMO is applied to Multi-User (MU) systems, however. For example, one problem that arises is how to efficiently organize response Transmit Opportunities (TXOPs) from several receivers. The receivers may have received downlink data in parallel from a Multi-User transmission (MU transmission), after which they may need to respond with a Block Acknowledgment (BA) frame or the like, possibly along with other uplink traffic. Conventional systems have relied on providing the addressed stations with a specified time slot after the downlink transmission, but this approach has several drawbacks. For example, the transmitting Access Point (AP) does not know the optimal length of the slot, because the uplink rate and the amount of data are not known to the AP. The rate could be specified by the AP, but this would typically result in too conservative of an estimate, and therefore too long response slots. Further, when the time slot information is not received by the STA, the time slot is wasted.